The present invention relates broadly to gaskets for providing electromagnetic interference (EMI) shielding and dust or other environmental sealing, and particularly to a gasket which is especially adapted for use within electronic devices and enclosures, and for other applications requiring a low closure force and cost-effective shielding solution.
The operation of electronic devices such as televisions, radios, computers, medical instruments, business machines, communications equipment, and the like is attended by the generation of electromagnetic radiation within the electronic circuitry of the equipment. As is detailed in U.S. Pat. Nos. 5,202,536; 5,142,101; 5,105,056; 5,028,739; 4,952,448; and 4,857,668, such radiation often develops as a field or as transients within the radio frequency band of the electromagnetic spectrum, i.e., between about 10 KHz and 10 GHz, and is termed xe2x80x9celectromagnetic interferencexe2x80x9d or xe2x80x9cEMIxe2x80x9d as being known to interfere with the operation of other proximate electronic devices.
To attenuate EMI effects, shielding having the capability of absorbing and/or reflecting EMI energy may be employed both to confine the EMI energy within a source device, and to insulate that device or other xe2x80x9ctargetxe2x80x9d devices from other source devices. Such shielding is provided as a barrier which is inserted between the source and the other devices, and typically is configured as an electrically conductive and grounded housing which encloses the device. As the circuitry of the device generally must remain accessible for servicing or the like, most housings are provided with openable or removable accesses such as doors, hatches, panels, or covers. Between even the flattest of these accesses and its corresponding mating or faying surface, however, there may be present gaps which reduce the efficiency of the shielding by presenting openings through which radiant energy may leak or otherwise pass into or out of the device. Moreover, such gaps represent discontinuities in the surface and ground conductivity of the housing or other shielding, and may even generate a secondary source of EMI radiation by functioning as a form of slot antenna. In this regard, bulk or surface currents induced within the housing develop voltage gradients across any interface gaps in the shielding, which gaps thereby function as antennas which radiate EMI noise. In general, the amplitude of the noise is proportional to the gap length, with the width of the gap having less appreciable effect.
For filling gaps within mating surfaces of housings and other EMI shielding structures, gaskets and other seals have been proposed both for maintaining electrical continuity across the structure, and for excluding from the interior of the device such contaminates as moisture and dust. Such seals are bonded or mechanically attached to, or press-fit into, one of the mating surfaces, and function to close any interface gaps to establish a continuous conductive path thereacross by conforming under an applied pressure to irregularities between the surfaces. Accordingly, seals intended for EMI shielding applications are specified to be of a construction which not only provides electrical surface conductivity even while under compression, but which also has a resiliency allowing the seals to conform to the size of the gap. The seals additionally must be wear resistant, economical to manufacture, and capability of withstanding repeated compression and relaxation cycles. EMI shielding gaskets and other electrically-conductive materials, their methods of manufacture, and their use are further described in U.S. Pat. Nos. 6,121,545; 6,096,413; 6,075,205; 5,996,220; 5,910,524; 5,902,956; 5,902,438; 5,882,729; 5,804,762; 5,731,541; 5,641,438; 5,603,514; 5,584,983; 5,578,790; 5,566,055; 5,524,908; 5,522,602; 5,512,709; 5,438,423; 5,202,536; 5,142,101; 5,141,770; 5,136,359; 5,115,104; 5,107,070; 5,105,056; 5,068,493; 5,054,635; 5,049,085; 5,028,739; 5,008,485; 4,988,550; 4,968,854; 4,952,448; 4,931,479; 4,871,477; 4,857,668; 4,800,126; 4,301,040; 4,231,901; and 3,758,123, in International Patent Appln. Nos. WO 00/23,513; 99/44,406; 98/54942; and 96/22672; Japanese Patent Publication (Kokai) No. 7177/1993; German Patent No. 19728839; Canadian Patent No. 903,020; in Severinsen, J., xe2x80x9cGaskets That Block EMI,xe2x80x9d Machine Design, Vol. 47, No. 19, pp. 74-77 (Aug. 7, 1975); in xe2x80x9cElectrically-Conductive Elastomers-ElectroCoat,xe2x80x9d Laird Technologies, Delaware Water Gap, Pa.; and in the following publications of the Chomerics Division of Parker Hannifin Corporation, Woburn, Mass.: xe2x80x9cSOFT-SHIELD(copyright) 1000 Series;xe2x80x9d xe2x80x9cSOFT-SHIELD(copyright) 2000 Series;xe2x80x9d xe2x80x9cSOFT-SHIELD(copyright) 4000 Series;xe2x80x9d xe2x80x9cSOFT-SHIELD(copyright) 5000 Series;xe2x80x9d and xe2x80x9cSOFT-SHIELD(copyright) 5500, Preliminary Product Data Sheet (1998) Series.xe2x80x9d
EMI shielding gaskets typically are constructed as a resilient core element having gap-filling capabilities which is either filled, sheathed, or coated with an electrically conductive element. The resilient core element, which may be foamed or unfoamed, solid or tubular, typically is formed of an elastomeric thermoplastic material such as polyethylene, polypropylene, polyvinyl chloride, or a polypropylene-EPDM blend, or a thermoplastic or thermosetting rubber such as a butadiene, styrene-butadiene, nitrile, chlorosulfonate, neoprene, urethane, or silicone rubber.
Conductive materials for the filler, sheathing, or coating include metal or metal-plated particles, fabrics, meshes, and fibers. Preferred metals include copper, nickel, silver, aluminum, tin or an alloy such as Monel, with preferred fibers and fabrics including natural or synthetic fibers such as cotton, wool, silk, cellulose, polyester, polyamide, nylon, polyimide. Alternatively, other conductive particles and fibers such as carbon, graphite, or a conductive polymer material may be substituted.
Conventional manufacturing processes for EMI shielding gaskets include extrusion, molding, or die-cutting, with molding or die-cutting heretofore being preferred for particularly small or complex shielding configurations. In this regard, die-cutting involves the forming of the gasket from a cured sheet of an electrically-conductive elastomer which is cut or stamped using a die or the like into the desired configuration. Molding, in turn, involves the compression or injection molding of an uncured or thermoplastic elastomer into the desired configuration.
Requirements for many shielding applications, such as involving access or door panels, back or face planes, I/O connector panels, and the like, shielding, specify a low impedance, low profile gasket which is deflectable under relatively low closure force loads, e.g., about 1.0-8.0 lbs per inch (0.2-1.5 kg per cm) of gasket length. Usually, a minimum deflection, typically of about 10%, also is specified to ensure that the gasket sufficiently conforms to the mating housing, board, panel, or other surfaces to develop an electrically conductive pathway therebetween.
As electronic devices continue to proliferate, it is believed that additional EMI shielding alternatives and options therefor would be well-received by the electronics industry.
The present invention is directed to a gasket construction for electromagnetic interference (EMI) shielding and dust or other environmental sealing, and which is especially adapted for use in low closure force applications. The gasket construction includes a resilient inner member and an electrically-conductive member which may be coated or otherwise bonded or applied as a layer to specified surfaces of the inner member.
In an illustrative embodiment especially suited for low closure force applications, the inner member is provided as a strip or sheet of an open or closed-cell polyurethane, silicone, neoprene, or other elastomeric foam or other resilient polymeric material. For improved tear resistance and strength, the foam may be reinforced, for example, with a layer of a polymeric film which is bonded to one or both of the upper or lower surfaces of the foam. A plurality of vias, which may be generally cylindrical, are formed through the thickness dimension of the foam.
The electrically-conductive member, which may be the cured film or other layer of an admixture of a resin, such as an epoxy, acrylic, polyurethane, or silicone, and one or more electrically-conductive particulate fillers, is coated on the top and bottom surfaces of the foam sheet or strip, or if a reinforcement is used, on the surface thereof, and also on the inner peripheral surfaces of the vias to provide electrical conductivity along a z-axis between the coating layers on the top and bottom surfaces of the foam. Thus, when compressed intermediate interfacing surfaces of an electronic device or other EMI shielded assembly, the gasket of the invention provides electrical continuity therebetween through the z-axis of the gasket as well as across the top and bottom surfaces of the foam.
Advantageously, by virtue of the z-axis conductivity being provided through the vias of the foam, the edges of the gasket need not be coated, wrapped, or otherwise covered with a conductive material to otherwise provide conductivity through the thickness dimension of the gasket. Accordingly, the gasket of the invention may be produced in sheets or strips which then may be die-cut or otherwise sectioned into complex shapes or patterns of any dimension, and including internal apertures, without the need to pre-size the width of the gasket or to coat or wrap the gasket after cutting in a separation operation. Thus, the gasket of the invention is particularly economical, and may be used in a variety of configurations, such as sheets, strips, pads, picture-frames, and die-cut patterns, thin or thick cross-sections, and shielding and/or grounding applications such as for door or access panels, card cages, vents, covers, PCMCIA cards, back or face planes, or I/O connector panels.
The present invention, accordingly, comprises the articles and methods of manufacture possessing the construction, combination of elements, and arrangement of parts and steps which are exemplified in the detailed disclosure to follow. Advantages of the present invention include a gasket construction which is economical to manufacture, and which may be produced in strip, sheet, pad or other form, and in a variety of sizes, thicknesses, and shapes gasket profile for a host of EMI shielding and/or grounding applications. These and other advantages will be readily apparent to those skilled in the art based upon the disclosure contained herein.